The present invention relates to digital converters, and more particularly, to a converter for digitizing the rotational output of a resolver.
A resolver is a type of rotary transformer typically having two stator windings and one rotor winding on an axis or shaft. A resolver is used for determining the position of the shaft or axis of the rotor, with respect to a reference point, within a space of one complete revolution of the shaft or a corresponding angular displacement within an angle space from 0° to 360°. The two stator windings are mechanically arranged such that their physical relation is shifted by a 90° angle. The physical spacing of the stator windings gives rise to a mathematical/electrical relation in that any signals induced in the stator windings from, for example, a rotor excitation signal, will be correspondingly shifted by 90° leading to the sine and cosine function relations assigned to the respective outputs of the stator windings. It will be appreciated that the degree of accuracy of the 90° physical relation based on, for example, accurate placement of stator windings, will affect the degree to which the outputs are electrically shifted by 90° and thus accurately represent a sine and a cosine relation respectively. It is understood by those of ordinary skill in the art that the sine and cosine functions are mathematically characterized by a 90° shift therebetween.
The amplification of a signal obtained by coupling with the stator windings is a function of the position of the resolver rotor axis and the relative position of the stator windings. Therefore, two types of output voltages (S3-S1, S4-S2) are modulated according to a sine wave function and a cosine wave function corresponding to the axis angle of the resolver rotor relative to the stator windings. The output voltage waveforms can be expressed in the following formulae (1) and (2).S3-S1=A(sin ωt)(sin θ)  (1)S4-S2=A(sin ωt)(cos θ)  (2)where “θ” is the angle of the rotor axis, “ω” is the angular velocity corresponding to the rotor excitation frequency (f), and “A” is the rotor excitation amplification.
Proposals have been set forth for Resolver/Digital (R/D) converters in which, of the continuous output voltage signals S3-S1 and S4-S2, the signal with the smaller absolute value or magnitude is divided by the signal with the larger absolute value and, based on a resulting continuous signal associated with the quotient, the angle data is obtained. An example of such a proposed R/D converter may be seen, for example, in Japanese Unexamined Patent Application Publication S62-38302. Therein, tan θ and cot θ are calculated from output voltages S3-S4, S4-S2. At the same time, a digital code, or the like, for A(sin ωt) is determined from the output voltage S3-S1, the digital code result such as an index or the like, and the angle section of the resolver rotor axis obtained from the digital code result or index. As noted, one of either tan θ or cot θ maybe used as a digital code, and angle data associated with resolver rotor axis angle θ and corresponding to the digital code or index and stored in advance in the table converter, is read and output.
Problems arise however, since the proposed R/D converter mentioned above requires angle data, in a range of 0° to 360° corresponding to the digital codes or indexes determined from output voltages, to be stored in the table converter. When high precision is required, memory capacity must be increased since a greater number of angle data values must be stored. In addition, the continuous calculation of quotients, namely tan θ and cot θ, with a microcomputer involves a long calculation time, thereby making it less desirable or effective in an operational environment where quantities associated with a high rate of rotation of the resolver rotor axis are present.